Bulk nickel silicides (NiSi and NiSi2) have been studied using the experimental positron lifetime and depth-resolved positron beam measurements. Ab-initio calculations of positron lifetime for the silicides have also been carried out using the atomic superposition method. For NiSi phase, it is found that the theoretically computed positron lifetime compares favourably with the experimentally deduced value indicating that NiSi is defect free. However, for NiSi2, the experimental positron lifetime does not compare well with the theoretical value suggesting that NiSi2 contains vacancy defects. This is further supported by the positron diffusion lengths deduced from the VEPFIT analysis of the positron beam results. (C) 2008 Elsevier B.V. All rights reserved.